There are several ways.
1.- You make a proposal
The client selects between all those who make a proposal and "Accept" the proposal of many of them
Then you receive access to chat message with the client
You (both ) agree to make an interview
Then , the client accept your proposal and you accept definetely and begin to work
2.- The client search a list of freelancers by knowledge, reputation and many things
The client select some of them and send invitations
you receive the invitation of the client and can accept or reject the invitation....
If you accept can begin to send / receive messages to set an interview